Hydrodynamic Fin

ABSTRACT

A hydrodynamic fin, such as a keel or a rudder of a watercraft, in particular a sailing craft or surfboard, which fin comprises at least two fin sections ( 20,21 ), which are each connectable to the watercraft ( 31 ) in a manner permitting rotation, in which the fin sections are rotatable in between two extreme positions around an axis ( 22,23 ) which is substantially parallel to the longitudinal symmetrical plane of the hull, in which extreme positions the fin sections provide a mainly cambered shape to the fin, with the characteristic that the fin sections can move with a substantially constant resistance from one extreme position to the other extreme position and the hydrodynamic fin is also provided with an end-stop ( 26 ) in order to stop a rotation of the fin sections when rotating from one extreme position to the other extreme position, such that the fin sections in use take either one extreme position or the other extreme position as a result of the hydrodynamic load.

The invention is related to a hydrodynamic fin, such as a keel, a dagger board or a rudder of a watercraft, such as a sailboat or surfboard according to the preamble of claim 1.

Such a fin is known from the American patent U.S. Pat. No. 5,181,678. In FIG. 14 in this American patent, a fin is shown, which is built from multiple rigid fin sections. The fin sections are joined together by means of hinges. The fin section forming the leading edge and the trailing edges are respectively provided with a round hole and a sliding slot. The fin can be mounted on two extending stationary rotation shafts, which are connected to the watercraft. The fin section at the leading edge with the round hole can rotate around one shaft. As a consequence of the presence of the sliding slot, the trailing edge fin section can rotate around the other shaft and slide relative to the shaft. In this way a freedom of movement exists to let the fin take a certain camber shape. The fin described also comprises elastic elements, in particular torsion springs. The elasticity of the elastic elements in the fin causes the camber of the fin to increase if the hydrodynamic load on the fin increases. The hydrodynamic load increases at a higher sailing speed. Therefore the fin camber will increase at an increased sailing speed. The amount of camber of the fin depends on the hydrodynamical force. This property of the fin is disadvantageous en does not make it's behavior satisfactory.

It is the object of the present invention is to at least partially eliminate one or more of the above drawbacks and/or to create a useable alternative.

This objective is achieved by a hydrodynamic fin, according to the preamble of claim 1 characterized in that the fin sections are moveable with a predominantly constant resistance from one extreme position to the other extreme position and the hydrodynamic fin is also provided with an end-stop in order to stop a rotation from one to the other extreme position, in such a way that the fin sections in use will take either one or the other extreme position as a result of a hydrodynamic load. This provides a hydrodynamic fin with an increased effectivity. The advantage enables a smaller design of the keel, which reduces the sailing resistance, and which enables a reduction of the draught of the watercraft, if desired. The degree of rotation of the fin sections is independent of the amount of the hydrodynamic force on the fin. This has the advantage that the fin according to the invention provides a substantially increased effectivity, starting at low speeds.

In a special embodiment of the invention, the leading edge and the back fin section are provided with a steel rotation shaft. The shafts can be mounted inside a connection head, or directly inside the hull of the watercraft, by means of rotation bearings. The bearings enable the rotation of the fin sections by an angle. An end-stop determines a maximum rotation of the fin sections, in such a way that a maximum camber of the fin is defined. A movement of the fin relative to the water will cause a hydrodynamic load on the fin. The hydrodynamic load causes the fin to adjust to the maximum camber shape. Amongst other things frictional resistance in the bearings will provide a resistance force against rotation of the fin sections. If the hydrodynamic load is larger than the force of resistance against rotation, the fin sections will rotate and take an extreme position. The bearings will cause a mainly constant frictional resistance when the fin section moves from one extreme position to the other. By a suitable choice of the bearings, the resistance force can be kept negligibly small and mainly constant. The resistance forces against the rotation of the fin sections are relatively small if compared to the hydrodynamic forces that occur at low speeds up to for example 5 knots. Because of this a maximum camber shape of the fin will occur already at low speed. The degree of rotation of the fin sections is independent of the amount of the hydrodynamic force on the fin. The advantage of this is that the fin according to the invention provides a substantially increased effectivity beginning at low speeds.

A keel, which is built from movable parts or rotating parts, and which therefore can take an asymmetric profile, is known from various patents, such as from the American patent U.S. Pat. No. 4,280,433 by Haddock.

Another drawback of known constructions of a fin with movable parts is the control mechanism, which is a disadvantage because of the vulnerability of such a mechanism, the extra space it takes, the attention it demands from the crew and the building costs and maintenance costs of the mechanism.

A second disadvantageous aspect of known fins with an adjustable camber of the profile, as in the mentioned patent, is that a large part of the fin is one fixed part of the watercraft. This limits the maximum extent of the camber.

Another disadvantage concerns the flexible side-plates for the deformation of the streamlined profile. A larger number of moving parts of the plates and the mechanism increase the vulnerability, which is disadvantageous. Also disadvantageous is that the desired deformation cannot be reached, for instance when dirt or growth of water plants occur behind the plates.

The hydrodynamic fin according to the invention comprises an end-stop, such that the rotation of a fin section is limited. This determines the extreme position. The end-stop can be constructed in many ways. As an example of a special design of the construction, a cylindrical pin-shaped end-stop can be mounted inside the connection head, such that the pin grips at one of the fin segments. The pin-shaped end-stop for instance extends into a hole in the finsection. The inside diameter of the hole is larger than the outside diameter of the cylindrical pin. When the fin section rotates, the inside surface of the hole will come into contact with the end-stop. It will be profitable if the position of the pin-shaped end-stop is made adjustable, such that the camber of the fin according to the invention becomes adjustable as desired. In this way for instance it is an advantage that the sail- and weather conditions can be taken into account.

A preferred embodiment of the fin according to the invention comprises two rigid fin sections, which are connected to each other. The rigid fin sections are connected by means of a hinge. In the cambered position of the profile or for the fin with a cambered shape, the length between the shafts, measured along the centerlines of the fin sections, is slightly larger than the distance between the 2 shafts. This over-determined condition is caused by the difference in length, which must be absorbed by means of loose clearance between the pin and the hole in the hinges. The hinge is constructed with a pin and slotted hole. The pin is fixed, for instance by means of a fitting with a negative diameter tolerance, on the first fin section. The second fin section is fitted with a slotted hole. The pin of the hinge extents into this slotted hole and can rotate inside the slotted hole and slide in longitudinal direction. Because of this hinge, the second fin section will follow the rotation of the first fin section. Because of this the fin according to the invention will take a cambered shape. The resistance against rotation, caused by the hinge is constant and relatively small relative to the occurring hydrodynamic loads and will not be any obstruction to let the fin take a cambered shape already at low speeds. The dimensions of the slotted hole can be made such that the end-surfaces form an end-stop. When the pin in the hinge comes into contact with an end-surface of the slotted hole, the fin sections will not be able to rotate any further, and the camber of the fin is hereby determined. The advantage is that in this way a stronger and lighter construction of the fin is obtained.

In another preferred embodiment according to the invention the gap in between two rigid fin sections is filled with a flexible material. In case of flexible hinges the over-determined condition during the movement from one to the other extreme position will be absorbed by means of stretching of the flexible soft material. This can be for instance silicon rubber or natural rubber. An advantage is that a smooth streamlined shape of the fin according to the invention is obtained by filling the gap. The flexible material is soft, therefore it hardly increases the resistance against rotation of the fin sections.

In a further application of the fin according to the invention a fixation system is foreseen. The fixation system comprises a cylindrical body with thread and is inserted in a hole with thread in the connection head. By turning the cylindrical body it will move downward and it will block the rotation of the fin sections.

If applied for instance in the keel or the dagger board of a sailboat, the invention results in better sailing performance, especially sailing up wind. However, it may occur that the asymmetric camber of the fin according to the invention is not desired. For instance at a down-wind course. In such case the freedom of movement can be blocked, in such a way that the fin is fixed in the symmetrical middle position. Preferably this is achieved by blocking the rotation of the two shafts which connect the fin to the watercraft by means of a securing pin, because these are accessible from the inside of the watercraft.

The hinges with a limited rotation and the two shafts which connect the fin to the watercraft, allow a rotation and a lateral shift of the fin sections, such that the fin can take a cambered shape with the desired asymmetrical profile automatically or in a self-adjusting way under the influence of the hydrodynamic load. In case of a keel or dagger board this load is caused by a sideward drifting of the watercraft, as a result of wind force and wave force on the hull of the watercraft, and wind force at the sail, if it concerns a sailing craft. In case of a rudder the load additionally occurs as a result of the rudder steering angle. It is known that the camber of a streamlined profile in a water flow leads to a much higher transverse force on the profile than a symmetrical streamlined profile, which is of course beneficial.

In addition to the automatically self-adjusting camber of the fin, the possibility exists to optimize the angle of attack of the water flow towards the fin by means of the positioning of the hinges and the two shafts. The self-adjusting property occurs under influence of the hydrodynamic loads. In case of a keel or dagger board this load results from a lateral drift of the watercraft, or in case of a rudder this is the result of the steering. The loads occur because the water pressure on the lee-side is higher than on the weather-side. In case of a rudder a higher water pressure occurs on the side where the rudder is moved. Of importance for the correct performance of the invention, the pressure difference should lead to a camber of the streamlined profile in the appropriate direction, i.e. convex on the weather-side, which is realized by a correct choice of the positions of the various hinge shafts and the two shafts with bearings. Therefore the two shafts are placed at the leading edge and back segment, such that the hinges, which connect the fin sections, are positioned in between these two. If in addition the shaft and bearing at the leading edge is placed sufficiently to the front, and the shaft and bearing at the trailing edge is placed sufficiently backward, a bending moment will be obtained that brings the fin in the correct camber. Another advantage of the fin according to the invention is that the angle of attack of the water flow can be influenced in favour by means of a suitable choice of the positions of the hinges and shafts and bearings.

The present invention also relates, among other things, to an application in a rudder. In this case the effect is important that the rudder angle can be smaller to generate an equal steering effect, which reduces the drag force as a result of steering action. In an application as a keel, the fin sections at the leading edge and the trailing edge, which are connected to each other, are connected to the hull of the watercraft by two shafts supported in bearings inside the hull of the watercraft. In an application as a rudder the two fin sections are not directly connected by two shafts to the watercraft, but to the top part of the rudder, which is connected to the watercraft by means of the main rudder shaft. The terminology “hinge” is to be understood as the common applications with a pin sticking in a hole to connect two parts in such a way that they can pivot (pin-hole hinge), e.g. like used often in rudder suspensions, or as an alternative flexible hinge. In the application of the invention in a rudder, in case of use of hinges in the version of pin-hole hinges, the section rotations will result in a dead-bend in the steering, because of which a small rudder angle will have no effect, which is undesirable. Therefore a rudder must have flexible hinges with certain stiffness against rotations, since this will take away the dead-bend. In addition a smooth outside surface will be obtained, without open gaps between the fin sections, thus the streamline will be optimal and the sailing resistance will be as low as possible.

The two shafts which connect the fin sections to the hull of the water craft must should not only enable a rotation of the fin sections relative to the watercraft and carrying the bending moments (a) and the transverse loads (b), they also have to take an axial load, more specific: the loads (c) downward resulting from the weight of the fin, loads (d) that occur in upward as well as in downward direction if the watercraft would run aground at an obstruction under water, or loads (e) upward that occur when ashore the weight of the watercraft is placed on the fin. An important requirement functioning properly is that the shafts can move without to too much friction under load a, b and c, such that the hydrodynamic loads are able to accomplish the rotations. With respect to the other loads the light and smooth rotation ability is of no importance. The shafts are therefore preferably fitted with a suitable combination of radial and axial bearings. The choice of this combination strongly depends on the size and weight of the fin. In case of a small and light fin this will be a combination of radial slide-bearings and an axial slide-bearings for both axial load directions, while for a larger and heavier construction roller-bearings for axial loads downward and the radial loads and slide-bearings for upward loads will be the best combination.

In case of a keel with an amount of ballast weight, intended to provide a stabilizing moment when the watercraft is at an inclined angle, the following complication will occur. As a result of the inclination of the watercraft the ballast weight will have a transverse component at the keel, which is acting in opposite direction of the hydrodynamic force. If this force would become larger than the hydrodynamic force, the asymmetry of the profile could be bent in the wrong direction. This phenomenon holds a limitation of the maximum ballast weight, which in many cases will be undesirable. In such case a provision will be applied which can lock the bearing of the two shafts in both asymmetrical positions, at the time the inclination is small enough and the profile is in the right position. Just like with the previous locking in the symmetrical middle position this will preferably be achieved by means of a securing pin which blocks the rotation of the two shafts that connect the fin to the watercraft, because these are accessible from the inside of the watercraft.

Further preferred embodiments are laid down in writing in the remaining sub conclusions.

Further explanation of the invention will be given by means of some application examples with three fin sections represented in figures below, which will give a practical embodiment of the invention, but may not be considered in a limiting sense, wherein:

FIG. 1 shows a side view of a watercraft with a keel and a rudder according to the invention;

FIG. 2 shows a top view of an application of a cross-section of a fin;

FIG. 3 shows a top view of an application as a rudder with a rudder angle; and

FIG. 4 shows a top view of an alternative construction of a hinge by means of the use of a flexible connection material, for instance rubber;

FIG. 5 a front view of an application as a fin for a surfboard;

The watercraft 1 with keel sections 2,3,4 are presented in FIG. 1. The three fin sections are connected to each other by means of two hinges 5, and the section at the leading edge and the section at the trailing edge are connected to the watercraft such that they can pivot by means of two shafts 6 fitted with radial and axial bearings within the watercraft. The fin sections of the rudder 8,9,10 are represented, connected to each other by means of two hinges 11, and the section at the leading edge and at the trailing edge are connected by rotation shafts to the top part of the rudder, which is connected to the watercraft by means of a rudder shaft 7.

FIG. 2 shows how the hinges 5 and the shafts 6 are positioned in the asymmetrical cambered profile. It also shows the limitation of the rotation of the hinges to a small angle caused by means of the small wedge-shaped gaps in between the fin sections. The arrow 14 points out the direction of the water flow at a small drift angle. The weather-side is in this case the topside and the lee-side is the bottom of the profile. The arrow 15 shows the resulting hydrodynamic transverse force and is pointed in the direction of the weather-side, i.e. this force compensates the external forces due to wind and waves, and therefore prevents drifting. The figure shows that if the external forces change to an opposite direction, for instance if the sailing craft turns through the wind from up-wind starboard course to up-wind portside course, the arrow 15 will switch to the opposite direction and thus change the cambering of the profile to the other direction.

FIG. 3 shows rudder sections 8,9,10 and the main rudder shaft 7. Arrow 15 represents the hydrodynamic transverse force resulting from the rudder angle a and eventual drifting of the watercraft. Arrow 16 is the projection of arrow 15 on to the sailing direction, and represents the extra sailing resistance of the rudder as a result of the rudder angle. The figure shows clearly that this force is approximately proportional with the rudder angle, so that a smaller angle leads to a smaller sailing resistance.

FIG. 4 shows an alternative hinge that allows a limited rotation between the fin sections 8 and 9 by means of the deformation of a flexible connection material 18, for instance rubber, and which has a resistance against rotation due to the stiffness of the flexible material and the selected thickness of material 18. The hinge is built from two u-shaped metal profiles 17 of a hard material, for instance steel. The U-profile has a ‘back’ and two ‘legs’. Between the backs of the U-profiles, which are placed in a mirrored position relative to each other, a rubber layer is glued or vulcanized. The thickness and flexibility or elasticity of the rubber layer are to be chosen such that the desired resistance against rotation of the hinge and the desired limitation of rotation will be obtained. The edge of the fin sections will be placed between the legs of the u-profile. To simplify mounting and dismounting the U-shaped profiles can be mounted to the fin sections by means of sunken bolts.

FIG. 5 shows an application according to the invention in the shape of a fin fit for a surfboard 31. The fin has two fin sections 20,21. The fin section 20 at the leading edge and the backward finsection 21 at the trailing edge are basically shaped like a plate. The fin sections are rounded-off to provide a streamlined shape.

The fin sections 20,21 are fitted with tapered rotation shafts 22 and 23. Here the shafts are made out of stainless steel. The shafts 22,23 are almost completely positioned within the fin sections, to transfer forces occurring during surfing. To manufacture a fin section, a mould is used which can be filled with epoxy resin. During the production of the fin section 20 at the leading edge, the shaft 22 was placed inside the mould containing the epoxy resin. After the hardening of the resin approximately ⅚ part of the length of the shaft 22, 23 is within finsection 20,21. The part of the shaft extending outside the fin section is cylindrical shaped and extends into a connection head 25. The shafts are positioned mainly parallel and in this case have an angle of approximately 75-85 degrees relative to the bottom surface 25 a of the connection head 25. Here, the connection head is of the type “tuttle box”. This type of connection head is a standard type for surfboards and has the dimensions to accommodate the shafts and bearings. Slide-bearings are used for the shafts 27. The shafts can rotate relative to the connection head 25. The fin sections 20,21 have a fixed connection to the shafts 22,23.

(blz.10) In this way the shafts 22,23 can rotate relative to the connection head 25, if the fin sections rotate.

The fin sections 20,21 are connected to each other by means of the hinges 29,30. The hinge 29 is is fitted with a pin which can rotate and is enclosed in a hole. The hinge 30 is placed near the bottom surface 25 a of the connection head 25. The diameter of the pin and hole in the hinge 30 are such that a small movement is possible, which allows the rotation movement of the fin sections. The hinge incorporates a pin, which extends into a slotted hole. Due to the slotted hole the pin has some freedom of movement to slide. The hinge 29 is placed near the tip of the shafts 22,23. The hole of hinge 29 is not a slotted hole that allows the pin of the hinge 29 to shift. Rotation of the fin sections is enabled by local bending of the fin. The gap between the fin sections is filled with a flexible material 24, such as silicon rubber. In this way, the fin obtains a smooth streamlined shape.

In the connection head 25 a fixation pin 26 between both shafts 22, 23 is placed. The fixation pin 26 is cylindrically shaped and provided with and external thread. The fixation pin is provided with a conic tip extending outside the connection head 25. The fixation pin 26 is placed in the connection head opposite to a hole in the finsection. This makes it possible to limit or block completely the cambering of the fin sections By adjusting the position of the fixation pin 26. 

1. A hydrodynamic fin, for a watercraft, having a hull, comprising: at least two fin sections, which are each connectable to the watercraft in a manner permitting rotation, wherein the fin sections are rotatable between a first position and a second position around an axis which is substantially parallel to a longitudinal symmetrical plane of the hull, and wherein, in the first and second positions, the fin sections provide a substantially cambered shape to the fin, and wherein the fin sections can move with a substantially constant resistance from one of the positions to the other position and the hydrodynamic fin further comprises an end-stop to stop rotation of the fin sections when rotating from one position to the other position, such that the fin sections in use take either one position or the other position as a result of the hydrodynamic load.
 2. A hydrodynamic fin according to claim 1, wherein the fin sections are connected to each other.
 3. A hydrodynamic fin according to claim 1, wherein the fin comprises two fin sections.
 4. A hydrodynamic fin according to claim 1, where the fin sections are mainly rigid.
 5. A hydrodynamic fin according to claim 1, wherein the fin sections are connected to each other by means of at least one hinges having a backlash.
 6. A hydrodynamic fin according to claim 5, wherein the hinge incorporates a pin and a slotted hole, and wherein the pin is connected to one of the fin sections and extends through a slotted hole that is made in another fin section.
 7. A hydrodynamic fin according to claim 6, wherein the pin of the hinge is slidable longitudinally inside the slotted hole.
 8. A hydrodynamic fin according to claim 6, wherein the slotted hole has end surfaces that form an end-stop for the pin in the hinge, wherein the distance between the end-surfaces of the slotted hole determines the first and second positions of the fin sections.
 9. A hydrodynamic fin according to claim 1, wherein a gap exists between the sections, which gap is filled with a soft flexible material.
 10. A hydrodynamic fin according to claim 9, wherein the soft flexible material is a silicon rubber.
 11. A hydrodynamic fin according to claim 1, wherein the fin comprises a pin which forms an end-stop, which acts at the fin section in the first or second position.
 12. A hydrodynamic fin according to claim 11, wherein the position of the pin can be adjusted, such that the first and second positions of the fin sections are adjustable.
 13. A hydrodynamic fin according to claim 1, wherein at least two fin sections have rotation shafts that are supported in bearings permitting rotation of the shafts inside a connection head.
 14. A hydrodynamic fin according to claim 13, wherein the connection head is of the tuttle-box type.
 15. A hydrodynamic fin according to claim 11, wherein the pin is mounted in a connection head.
 16. A hydrodynamic fin according to claim 1, wherein the fin comprises a controllable fixation system to lock the fin sections in a position in between the first and second positions.
 17. A hydrodynamic fin according to claim 13, wherein the two rotation shafts supported in bearings are fitted with a fixation system such that at least one of the fin sections may be selectively fixed in any one of the first position, the second position and a position between the first and second positions.
 18. A hydrodynamic fin according to claim 5, wherein the hinge between the fin sections is constructed from two profiles having a U-shape and made of a hard material, which are mounted at the fin sections, between which a flexible material is glued or vulcanized.
 19. A watercraft provided with a fin according to claim
 1. 